Radio frequency (RF) equipment have used a variety of approaches and structures for receiving and transmitting radio waves in selected frequency bands. The type of filtering structure used often depends upon the intended use and the specifications for the radio equipment. For example, dielectric filters may be used for filtering electromagnetic energy in the ultra-high frequency (UHF) band, such as those used for cellular communications in the 800+ MHz frequency range. Typically, such filter structures are implemented by coupling a number of dielectric resonator structures together. One can also use metal coaxial resonators in such filters are coupled together via capacitors, inductors, or by apertures in walls separating the resonator structures. The number of resonator structures used for any particular application also depends upon the system specifications and, typically, added performance is realized by increasing the number of intercoupled resonator structures.
However, because of an increase in the number of users utilizing a limited bandwidth, demand has increased for greater frequency selectivity than can be provided by normal or non-superconducting resonator filters, especially for RF signals in the ultra-high frequency bands used for cellular communications. High frequency selectivity has previously been accomplished using High Temperature Superconducting (HTS) filters, usually as front-end filters for cellular base station receivers. However, HTS front-end filters may be susceptible to failure, or degradation in performance, induced by lightning surges or other high power signals. In addition, the non-linearity of HTS filters produces in-band intermodulation spurious signals from out-of-band interferers.
For cellular or similar base stations, typical lightning protectors have only one resonator and do not provide sufficient protection from high power co-located radio frequency signals originating from the transmit side of the base stations. These co-located transmission signals are especially troublesome because they are relatively closely spaced to the operating frequency of the base station receivers. Accordingly, there is a need for a filter that overcomes the above-mentioned and other disadvantages associated with the prior art.